1. Field of the Invention
The present invention relates to a method and catheter for treating biological tissues with light activated drugs, and more particularly, to a method and catheter for treating biological tissues by delivering a light activated drug to a biological tissue and exposing the light activated drug to ultrasound energy.
2. Description of Related Art
It is frequently desirable to kill targeted biological tissues such as tumors and atheroma. One technique for causing targeted tissue death is called photodynamnic therapy which requires the use of light activated drugs. Light activated drugs are inactive until exposed to light of particular wavelengths, however, upon exposure to light of the appropriate wavelength, light activated drugs can exhibit a cytotoxic effect on the tissues where they are localized. It has been postulated that the cytotoxic effect is a result of the formation of singlet oxygen on exposure to light.
Photodynamic therapy begins with the systemic administration of a selected light activated drug to a patient. At first, the drug disperses throughout the body and is taken up by most tissues within the body. After a period of time usually between 3 and 48 hours, the drug clears from most normal tissue and is retained to a greater degree in lipid rich regions such as the liver, kidney, tumor and atheroma. A light source, such as a fiber optic, is then directed to a targeted tissue site which includes the light activated drug. The tissues of the tissue site are then exposed to light from the light source in order to activate any light activated drugs within the tissue site. The activation of the light activated drug causes tissue death within the tissue site.
Several difficulties can be encountered during photodynamic therapy. For instance, since the light activated drug is typically administered systemically, the concentration of the light activated drug within the targeted tissue site is limited by the quantity of light activated drug administered. The concentration of the light activated drug within a tissue site can also be limited by the degree of selective uptake of the light activated drug into the tissue site. Specifically, if the targeted tissue site does not selectively uptake the light activated drug, the concentration of light activated drug within the tissue site can be too low for an effective treatment.
An additional problem associated with photodynamic therapy concerns depth of treatment. Light cannot penetrate deeply into opaque tissues. As a result, the depth that light penetrates most tissue sites is limited. This limited depth can prevent photodynamic therapy from being used to treat tissues which are located deeply in the interior of a tissue site.
There is currently a need for a method and apparatus which can be used to cause death to tissues death deep within a tissue site. When the method and apparatus employ light activated drugs, the method and apparatus should be able to provide an appropriate concentration of light activated drug within the tissue site.